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Vlad's Analysis of the H-NMR of Salicylic acid
By Vlad Limonnik
Figure 1.1- Salicylic Acid (2-hydroxybenzoic acid)
[How do you conclusively assign H3 vs H6 and H4 vs H5? (hint you can look at the spectra of other compounds) JCB]
I looked at a functional group priority list and saw that Carboxylic acids have higher priorities than hydroxy groups, therefore I numbered the COOH as #1 and the OH as #2.
[I'm not asking about the priority for numbering - I'm asking why you assigned specific peaks to the number you drew JCB]
I saw that H3 was ~6.920 ppm, H6 was ~7.800 ppm, and that they were both doublets. I then assumed that since H6 was closer to the carboxylic group (which is strongly downfield and would cause H6 to also shift more downfield) that H6 should be more downfield than H3.
[You can't assume that - the way to do it is to compare against the NMR of compounds that have only a phenolic or only a carboxylic acid group on the ring JCB]
Figure 1.2- H-NMR Spectrum of Salicylic Acid; scale in ppm (delta scale)
Figure 1.3- One sharp peak at ~2.5 ppm is seen from the DMSO that was used
Figure 1.4- A triplet is seen ~6.870, ~6.900, ~6.910 ppm. This corresponds to the H that is bonded to C-5. A doublet is seen at ~6.925 ppm and ~6.940 ppm, which is consistent with the H that is bonded to C-3. The values are ~7.000 ppm because the H's are located on an aromatic ring, which is consistent with having a ppm of 7-8.
Figure 1.5- A triplet is seen ~7.450, ~7.465, ~7.480 ppm. This corresponds to the H that is located at C-4. The values are ~7.000 ppm because the H's are located on an aromatic ring, which is consistent with having a ppm of 7-8.
Figure 1.6- A doublet is seen ~7.780 ppm and ~7.810 ppm. This corresponds to the H that is located on C-6. The values are ~7.000 ppm because the H's are located on an aromatic ring, which is consistent with having a ppm of 7-8.
Figure 1.7- A broad absorption is seen ~10.50 ppm - ~12.20 ppm, which is consistent with COOH and typically has broad absorptions ~10 ppm- ~12 ppm.
Figure 1.8- A broad absorption is seen ~11.500 ppm, due to the OH that is located on C-2.
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